Method for fabricating a substrate with useful layer on high resistivity support

ABSTRACT

A substrate suitable for producing a high frequency electronic circuit. This substrate includes a support substrate having a controlled amount of interstitial oxygen and which is treated to precipitate at least some of the oxygen therein; and a useful layer supported by the support substrate. Advantageously, the support substrate has high resistivity and includes oxygen precipitates beneath the useful layer while also being free of depleted zones of oxygen precipitates adjacent the useful layer. This is prepared by the methods disclosed herein which are applicable in particular to SOI substrates.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 10/968,695 filedOct. 18, 2004, now U.S. Pat. No. 7,268,060, which is a continuation ofInternational application PCT/IB03/02237 filed Apr. 23, 2003, the entirecontent of each of which is expressly incorporated herein by referencethereto.

BACKGROUND

The present invention generally relates to the manufacture of compositesubstrates based on semiconductors and in particular, those based onsilicon, such as “SOI” (Silicon On Insulator) type substrates and evenbulk silicon substrates.

More particularly, the invention aims to produce a substrate formed froma thin layer of single crystal silicon of suitable quality, or othersemiconductor, on a solid base of high resistivity, typically greaterthan 1 KΩ·cm (the thin layer may be of any resistivity), such asubstrate being described as “semi-insulating”.

These substrates are used, for example, in microwave applications, whereone wishes to integrate active and passive components (in particularinductances) on the same substrate.

Recourse to semi-insulating substrates to produce circuits operating athigh and very high frequencies (typically of the order of severalgigahertz for so-called “MMIC” (Microwave Integrated Circuit) circuits),limits high frequency losses due to Foucault currents in the mass of thesupport. This is why the support is conventionally made from galliumarsenide AsGa, known for its good intrinsic properties and highresistivity, typically of the order of 10 MΩ·cm.

Another justification for the use of highly resistive substrates forsuch circuits is associated with the manufacture of integrated passivecomponents forming part of these circuits, whose quality factor must beas high as possible at the frequencies concerned. Now, for very highfrequencies, the quality factor drops in the presence of conducting orsemiconducting elements in the neighborhood of the component.

The concern above is to allow for this, consisting, in classical SOIsubstrates, in producing a very thin insulating layer between the usefulpart of the substrate and the remainder of it. In fact, it is a questionin this context of eliminating as far as possible any conducting partsat distances of a few hundred μm or more and thereby obtaining highresistivity throughout the thickness of the support.

In terms of substrates with an insulating support, “SOQ” (Silicon OnQuartz) substrates are equally well known, and even “SOS” (Silicon OnSapphire) (Al₂O₃), where the support has the required high resistivity.However, such substrates may be difficult to obtain with good quality,often being contaminated with metallic impurities.

A good quality thin layer of silicon may also be difficult to obtain,despite whatever well known technology is used to manufacture it(hetero-epitaxy, forming of layer with bonding principally by molecularbonding). Associated problems can arise from the use of such substrates,in particular because certain manufacturers of integrated circuits mayrefuse to introduce them into their production line, either for reasonsof contamination, or because their transparency to light makes themincompatible with some optical sensors used in industrial processes, inparticular for metrology purposes, or even because the maximumtemperature to which they can be exposed without damage makes themincompatible with certain processes.

It will also be noted that SOQ or SOS substrates of good quality are notavailable in large diameters.

Another approach could be to produce high frequency integrated circuitsin substrates possessing a silicon support layer with normal resistivity(either solid silicon or SOI substrates), then to transfer thesecircuits to insulating supports. Such techniques could firstly becumbersome, implying a double turnaround of the circuit, and secondlyunsuitable for an industrial process (high cost price, in particularbecause of the loss of the original supporting layer or the necessity toremove this, and low yield).

Yet another approach might consist in using solid FZ (Float Zone)silicon substrates. The main problem with these substrates is that theyare not available today in diameters greater than 150 mm. Anotherproblem is their low residual oxygen content, which firstly makes themmechanically fragile and secondly, by limiting the formation of oxygenprecipitates, limits their ability to trap metallic impurities(“gettering”).

In addition, the production of high resistance substrates in solid CZsilicon with high resistivity could be considered using a variant of theCZ technique known under the name of MCZ (“Magnetic Field Applied CZ”,or “CZ in a magnetic field”), which, however, would involve sacrificingthe crystallographic quality of these substrates, making them unsuitablefor the production of components in them. In particular, the crystallinequality becomes mediocre, in particular due to the presence of an excessof oxygen precipitates, a mass of holes-known under the acronym of COPS(Crystal Originated Particles), which would prevent the production ofhigh quality circuits with an acceptable yield.

It will also be noted that in solid substrates of FZ or CZ silicon, theuseful layer and the substrate mass forming the support for the usefullayer would come from the same slice or the same ingot of silicon, whichpresents a degree of constraint for the designers. In particular, whilethe presence of faults in the mass of the ingot does not in general poseinsurmountable problems, the presence of such faults in the neighborhoodof the surface of the useful part of a substrate cut from such an ingotmakes it unsuitable for its purpose and the rate of rejection is high.

Finally, one could consider producing such high frequency circuits onSOI substrates whose support layer would be in FZ silicon, or even on asubstrate whose useful layer would be formed by bonding onto such a FZsilicon support layer, in order to obtain the same decoupling betweenthe useful layer and support layer. However, one would still haveproblems, in particular the limitation of the diameter to 150 mm today.

In addition, PCT application WO00/55397 describes a process for themanufacture of a substrate in silicon starting from an CZ silicon ingotwith a particular interstitial oxygen content 01, then using a thermalprecipitation treatment, reducing the interstitial oxygen content toobtain a substrate that maintains a high resistivity while presentinggrains of oxygen precipitates (suitable for the purpose of trappingmetallic impurities) and suitable mechanical strength.

This document also describes the production of a substrate of the SOItype by the bonding of a useful layer onto such a high resistivitysubstrate and in this case the thermal precipitation treatment ispreferably performed after bonding and also serves to strengthen thebonding interface. This technique however has certain problems. In thefirst place, it may in some cases be required not to subject the usefullayer to excessive temperatures (particularly with useful layers ingallium arsenide AsGa or in indium phosphide InP, or even useful layersin pre-treated silicon implying limitations relating to the subsequentexposure to high temperatures) and such materials may then beincompatible with this known process.

Secondly, the fact that heat treatment necessary for precipitation mustalso be designed in the perspective of reinforcement of the bondinginterface results in an obvious lack of flexibility, knowing that itcould be difficult to produce a satisfactory compromise between theadequate treatment for the work on the bonding interface and adequatetreatment for precipitation and stabilization. It must therefore benoted in this regard, the behavior of oxygen in the field of electricaldonor generation and precipitation and growth of precipitant grainsvaries especially as a function of conditions (duration and temperature)of the different phases of heat treatment.

Finally and most importantly, in the case of an SOI, the interstitialoxygen content Oi and its behavior in the vicinity of an insulting layerof oxygen separating the base of the useful layer are poorly controlled,especially as concerns the way in which the oxygen is capable ofoutwardly diffusing into the oxide layer. Consequently, one riskshaving, without these phenomena being capable of being controlled:either an Oi content that is locally too low to obtain satisfactoryformation of precipitates; or, on the other hand, an Oi that is locallytoo high, leading to a significant lowering of resistivity and/or to astability defect in this resistivity (which could, in particular, dropat the time of subsequent exposure to elevated temperatures, for exampleduring realization of the components).

Accordingly, improvements in these methods and substrates are desired.

SUMMARY OF THE INVENTION

The present invention relates to a method for fabricating a substratecontaining a useful semiconductor layer on a support having a highresistivity suitable for producing high frequency electronic circuits,which comprises: preparing a base substrate made of a semiconductormaterial that includes a controlled amount of interstitial oxygen; heattreating the substrate to precipitate some of the interstitial oxygentherefrom; removing a superficial layer from a surface of the basesubstrate that is intended to receive a useful layer, wherein thesuperficial layer is oxygen depleted or includes undesirable electrondonors due to the heat treating; and forming the useful layer on thebase substrate surface, wherein the base substrate acts as a support forthe useful layer.

This method is particularly applicable to SOI substrates having highresistivity for use in forming high frequency electronic circuits.

Another embodiment of the invention relates to a substrate suitable forproducing a high frequency electronic circuit. This substrate comprisesa base or support substrate that includes a controlled amount ofinterstitial oxygen and is treated to precipitate at least some of theoxygen therein; and a useful layer supported by the support substrate.Advantageously, the support substrate has high resistivity and includesoxygen precipitates beneath the useful layer while also being free ofdepleted zones of oxygen precipitates adjacent the useful layer.

The support substrate may be a semiconductor of the SOI-type and have aconcentration of interstitial oxygen that is approximately 15×10¹⁷/cc orhigher prior to oxygen precipitation. The useful layer preferablycomprises a material selected from the group consisting of silicon,silicon-germanium, gallium arsenide, indium phosphide, silicon carbideand gallium nitride. In another preferred embodiment, the substratecomprises an insulating layer located between the useful layer and thesupport substrate.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

Other features and advantages of the present invention will become moreapparent when reading the following detailed description of a preferredembodiment, provided as a non-limiting example, with reference to theattached drawing, wherein:

FIG. 1 is a schematic of the principal steps of an exemplary process formanufacturing substrates according to the invention; and

FIGS. 2A-2F diagrammatically represent in transverse section thedifferent states of materials in a particular case of implementation ofthe process of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention now provides a solution to the limitations of theprior art. It specifically provides, in the context of realization ofsubstrates implementing an operation for forming a useful layer on asupport, a manufacturing method for a substrate with a base layer havingelevated resistivity, wherein at least one of the following advantagescan be achieved:

preserving and stabilizing the natural resistivity of the material usedfor the support, including in the vicinity of the useful layer;

assuring the surface qualities enabling satisfactory deposition orbonding of the useful layer;

assuring and stabilizing the qualities of the support in the vicinity ofthe useful layer especially in terms of trapping impurities, and

obtaining a substrate having good mechanical properties.

Accordingly, the present invention proposes a method for fabricating asubstrate of SOI-type containing a useful semiconductor layer on asupport having a high resistivity suitable for producing high frequencyelectronic circuits, by the following successive steps: (a) preparing abase substrate made of semiconductor material comprising a controlledquantity of interstitial oxygen; (b) conducting a heat treatment definedso as to produce at least partial precipitation of the interstitialoxygen of the base substrate; (c) on one surface of the base substratefor receiving a useful layer, eliminating a superficial layer to acontrolled depth; (d) forming the useful layer on the surface of thebase substrate, the latter forming the support for the useful layer.

Certain preferred non-limiting features of this process include thefollowing. The base substrate is preferably formed by fabrication of asilicon ingot by the MCZ method and cutting out a portion of this ingotfor use as the base substrate.

The concentration of interstitial oxygen in the base substrate isgreater than or equal to approximately 15×10¹⁷/cc.

Formation of the useful layer is preferably carried out by layertransfer from a donor plate or wafer. This can be performed by creatingin a donor wafer, by implantation of gas species, a weakened zonelocated under the surface and intended for delimiting the useful layer,bonding the wafer onto the base substrate at the surface, and detachingthe useful layer relative to the rest of the donor wafer by applicationof stresses at the weakened zone. These stresses are typically ofthermal and br mechanical origin, as is generally known in the art. Thedonor wafer is preferably made of a material chosen from the groupconsisting of silicon, silicon-germanium, gallium arsenide, indiumphosphide, silicon carbide, and gallium nitride.

Step (b) can be implemented to also provide stabilization of theprecipitated oxygen. To do this, step (b) preferably comprises at leasttwo, and preferably three, heat treatments at different temperatures,with the heat treatments being carried out in progressively increasingtemperature ranges.

Step (c) is common to at least one polishing operation applied to thebase substrate surface. In particular, step (c) comprises an operationchosen from the group consisting of mechano-chemical polishing, dry orwet etching, and sacrificial oxidation. This is done to remove athickness of approximately between 0.5 μm and 10 μm.

The method also comprises, prior to step (d), a step for forming aninsulating layer on the useful layer and/or on the base substrate, withthis insulating layer being disposed between the useful layer and thebase substrate after step (d).

With reference to the Figures, initially a base substrate 10 (FIG. 2B)is prepared for forming a support layer having elevated resistivity,typically of 1,000 ohm·cm or more, using an SOI substrate forradiofrequency or microwave circuits.

For this purpose, an MCZ silicon ingot 1 (FIG. 2A) is grown using theclassical technique (step 100), following which said ingot is cut instep 110, for example, by sawing into slices 10 of a given thickness(FIG. 2B). For example, a typical slice has a diameter of 200 μm and athickness of 725 μm. Each of these slices forms a base substrate as willbe seen below. It should be noted at this point that the crystalcomprises a relatively significant quantity of interstitial oxygen,typically of the order of 15×10¹⁷/cc or more.

Heat treatment (step 120) is then applied to the base substrate 10 forproducing precipitation of the interstitial oxygen and stabilizing theprecipitates; this being done for the purpose of giving the basesubstrate the desired highly resistant character for ensuring nucleationphases (formation of the nuclei of precipitates) and growth of thenuclei, comprises a succession of phases at progressively increasingtemperatures and over defined periods of time. For example, useful heattreating conditions include: 650° C. for 2 hours, preferably 800° C. for2 hours, and most preferably 1000° C. for 4 hours.

These parameters can, however, vary widely while preferably respectingthe following rule. A temperature progression must be assured—theprogression must comprise marked steps or even phases, during which thetemperature rise is very slow. The temperature at the start of treatmentis between approximately 400 and 650° C., while the temperature at theend of treatment is between approximately 1000° C. and 1100° C. Itshould be recognized that if a certain temperature is exceeded,typically between 1100 and 1200° C., there is the risk of abruptlydissolving the oxygen precipitates that are formed.

At the end of this precipitation annealing operation, the substrate 10is subjected, on its surface intended for receiving a thin usablesurface as will be detailed in the following, to a polishing step 130for obtaining in the usual fashion a surface roughness that issufficiently low to allow good bonding quality by molecular adhesionthat is carried out later. It has been observed, however, according toone feature of the invention, that such polishing, if it were used forremoving a quantity of material greater than that removed with classicalpolishing, would have other appreciable advantages. More precisely,removal of the material of a certain thickness, which results from suchpolishing, contributes to significantly improving the quality of thebase substrate in the vicinity of the surface that will receive the thinuseful layer.

In fact, it has been found that at the end of the precipitationannealing operation, the behavior of the oxygen in the zone situated inthe vicinity of the exposed surface of the base substrate that is toreceive the useful layer was poorly controlled. More precisely, thefollowing poorly controlled phenomena can be seen in this zone inconnection with the heat treatment. Exodiffusion of oxygen, whichresults in a zone depleted of oxygen and, consequently, depleted ofoxygen precipitates contributing to trapping of impurities (commonlyknown as the denuded zone). Poor stabilization of the interstitialoxygen, where the Oi atoms being capable of regrouping with other atomsto form electrical donors capable of lowering considerably, by theuntimely doping that they bring about, the resistivity of the supportlayer. Thus, in realizing an elimination of the base substrate materialat this zone by polishing, the aforementioned problems are resolved,because one then obtains a zone of support material for receiving theuseful layer. This zone is only minimally affected or not at allaffected by the aforesaid phenomena.

Accordingly, the structure obtained will have advantageous qualities.Principally the stability of the residual interstitial oxygen thuspreserving and stabilizing electrical resistivity. Secondarily, topreserve oxygen precipitate content contributing to the trapping effect.Finally, to preserve structural uniformity and mechanical resistance ofthe substrate.

Removal of the aforesaid material is implemented in one or more stepsusing one of the methods available on the market or a combinationthereof. One can mention in particular the mechano-chemical polishingtechniques, dry chemical etching (by plasma) or wet chemical etching(etching bath) or even the sacrificial oxidation techniques (oxidationof the material at a controlled depth followed by selective chemicaletching of the oxide) in combination with other techniques.

In one preferred embodiment, a mechano-chemical polishing step is usedfollowed by a finishing step by chemical etching or sacrificialoxidation or even a chemical etching step followed by a mechano-chemicalpolishing step. Preferably, polishing is done to globally remove athickness of material on the order of 0.5 μm to 10 μm, this value beingchosen on the basis of experience as a function of the scope of theaforementioned phenomena.

Then, the polished surface can be (in a manner not shown in FIG. 1), ifnecessary, subjected to a step for forming a classical insulating layerin itself (layer 30′, 30″); in FIG. 2B).

In the case of a species, where the useful layer of the final SOIsubstrate is produced by a layer transfer technique such as theSMART-CUT® process, a donor substrate 20 (FIG. 2C), typically comprisedof monocrystalline silicon, in its turn, is subjected in classicalfashion to thermal oxidation (step 200) or even to oxide deposition, forforming an oxide layer 30″, then to a step for creating a weakened zone21 (FIG. 2D) at a given depth under the oxide layer (not shown),preferably by implantation of gas species (step 210); this weakened zonedelimits a thin layer 22 on the oxide side relative to the rest 23 ofthe donor substrate 20 and this thin layer constitutes the useful layer.

Bonding by molecular adhesion is then carried out (step 140; FIG. 2E) ata bonding interface between the oxide layers 30′, 30″ of the basesubstrate 10 and the donor substrate 20 so that in the end they form theburied oxide 30 of the SOI substrate (FIG. 2F).

Heat treatment in one or more steps is then done (steps globallyindicated at 150); this treatment being intended essentially toparticipate in the detachment of the weakened zone 21; to reinforce thebonding interface; to stabilize and improve the crystalline structure ofthe SOI substrate formed; and, if required, to carry out the oxidationphase of a finishing step by sacrificial oxidation.

In this respect, FIG. 2F represents the situation after completedetachment along the weakened zone 21. On one side the desired highresistivity SOI substrate is obtained, with the layer 10 forming thesupport and the thin layer 22 forming the useful layer, the oxide 30produced on this latter is situated at the bonding interface forming theinsulator.

If required, these steps can also play a role in the precipitation of aninterstitial oxygen remnant situated in the support 10 and notprecipitated at the time of the precipitation annealing operation, oreven in stabilization, principal or supplementary, of the precipitatesformed at the time of said specific annealing.

Obviously, the present invention is in no way limited to the embodimentdescribed above and represented in the drawings. In particular, itapplies to SOI substrates in which the support layer has a targetresistivity that can vary widely and be made of diverse semiconductormaterials.

One can mention especially SOI substrates comprising a useful layer ofsilicon-germanium, gallium arsenide, indium phosphide on ahigh-resistivity silicon support, substrates that are particularly usedfor producing high-frequency circuits. Useful layers can also beprovided that are made of germanium, gallium nitride, silicon carbide,etc.

It should be noted at this point that for realizing useful layers madeof gallium arsenide or indium phosphide, it is particularly important tocarry out the essential step of heat treatment for precipitating theoxygen and stabilizing it in the Si support prior to bonding, becausethese materials must not be exposed to very high temperatures.Typically, these temperatures must not exceed 300 to 400° C. if thematerials are not protected and 700 to 800° C. if the materials arecovered with a protective layer (SiO₂, for example) or if annealing isdone under a specific atmosphere (arsine, phosphine, etc.).

Furthermore, the present invention applies also to the case, where theusable thin layer is applied to the support by various thin layerapplication techniques (typically the “bond and etch back” layerapplication techniques or even the type commonly known by the commercialname ELTRAN, or further still in the case, where the thin useful layeris formed directly on the support, by epitaxy for example, thepreparation of the support being similar in this instance.

1. A substrate suitable for producing a high frequency electroniccircuit, the substrate comprising: a support substrate that has asurface and includes a controlled amount of interstitial oxygen and istreated to precipitate at least some of the oxygen therein; and a usefullayer supported by and in contact with the support substrate; whereinthe support substrate has high resistivity and includes oxygenprecipitates beneath the useful layer and treated to be free of depletedzones of oxygen precipitates adjacent the surface of the supportsubstrate at the interface with the useful layer.
 2. The substrate ofclaim 1, wherein the support substrate is a semiconductor of theSOI-type.
 3. The substrate of claim 1, wherein the support substrate hasa concentration of interstitial oxygen that is approximately 15×10¹⁷ /ccor higher prior to oxygen precipitation.
 4. The substrate of claim 1,wherein the useful layer comprises a material selected from the groupconsisting of silicon, silicon-germanium, gallium arsenide, indiumphosphide, silicon carbide and gallium nitride.
 5. The substrate ofclaim 1, wherein a portion of the useful layer comprises an insulatinglayer, wherein the insulating layer is in contact with the support layerat the interface.
 6. The substrate of claim 1, wherein a portion of thesupport substrate comprises an insulating layer which is in contact withthe useful layer at the interface.